In the search for extraterrestrial intelligence, scientists are often accused of "carbon chauvinism" because they expect other lifeforms in the universe to be made up of the same biochemical building blocks as we do, tailoring their searches accordingly. But life may well be different - and people are thinking about it - so let's explore ten possible biological and non-biological systems that expand the definition of "life."
And after reading, you will say which form is questionable for you, even theoretically.
Methanogens
In 2005, Heather Smith of the International Space University in Strasbourg and Chris McKay of NASA's Ames Research Center prepared a paper examining the possibility of life based on methane, the so-called methanogens. Such life forms could consume hydrogen, acetylene and ethane, exhaling methane instead of carbon dioxide.
This could make possible habitable zones for life in cold worlds like Saturn's moon Titan. Like Earth, Titan's atmosphere is mostly nitrogen, but mixed with methane. Titan is also the only place in our solar system, besides the Earth, where there are large liquid bodies of water - lakes and rivers of an ethane-methane mixture. (Underground bodies of water are also present on Titan, its sister moon Enceladus, and Jupiter's moon Europa.) Liquid is considered essential for molecular interactions in organic life, and of course the focus will be on water, but ethane and methane also allow such interactions to occur.
NASA and ESA's Cassini-Huygens mission in 2004 observed a dirty world with a temperature of -179 degrees Celsius, where the water was as hard as rock, and methane floated through river valleys and basins into polar lakes. In 2015, a team of chemical engineers and astronomers at Cornell University developed a theoretical cell membrane made of small organic nitrogen compounds that could function in Titan's liquid methane. They named their theoretical cell "nitrogenosome", which literally means "nitrogenous body", and it had the same stability and flexibility as the earth's liposome. The most interesting molecular compound was the acrylonitrile azotosome. Acrylonitrile, a colorless and toxic organic molecule, is used for acrylic paints, rubber and thermoplastics on Earth; it was also found in the atmosphere of Titan.
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The implications of these experiments for the search for extraterrestrial life are difficult to overestimate. Life not only could potentially develop on Titan, but it can also be detected by hydrogen, acetylene and ethane traces on the surface. Methane-dominated planets and moons may not only be around Sun-like stars, but also around red dwarfs in the broader Goldilocks zone. If NASA launches the Titan Mare Explorer in 2016, we will have detailed information about the possible life on nitrogen in 2023.
Silicon based life
Silicon-based life is perhaps the most common form of alternative biochemistry, beloved by popular science and fiction - remember the Horta from Star Trek. This idea is far from new, its roots go back to the reflections of H. G. Wells in 1894: “What fantastic imagination could run out of such an assumption: imagine silicon-aluminum organisms - or, perhaps, silicon-aluminum people at once? - which travel through an atmosphere of gaseous sulfur, let's say, on seas of liquid iron with a temperature of several thousand degrees or something like that, just above the temperature of a blast furnace.
Silicon remains popular precisely because it is very similar to carbon and can form four bonds, like carbon, which opens up the possibility of creating a biochemical system completely dependent on silicon. It is the most abundant element in the earth's crust, aside from oxygen. There are algae on earth that incorporate silicon into their growth process. Silicon plays a second role after carbon, because it can form more stable and diverse complex structures necessary for life. Carbon molecules include oxygen and nitrogen, which form incredibly strong bonds. Silicon-based complex molecules, unfortunately, tend to disintegrate. In addition, carbon is extremely abundant in the universe and has been around for billions of years.
Silicon-based life is unlikely to emerge in an earth-like environment, since most of the free silicon will be trapped in volcanic and igneous rocks of silicate materials. It is believed that in a high-temperature environment, everything may be different, but no evidence has yet been found. An extreme world like Titan could support silicon-based life, possibly coupled with methanogens, since silicon molecules like silanes and polysilanes can mimic Earth's organic chemistry. However, Titan's surface is dominated by carbon, while most of the silicon is deep below the surface.
NASA astrochemist Max Bernstein suggested that silicon-based life could exist on a very hot planet, with an atmosphere rich in hydrogen and poor in oxygen, allowing complex silane chemistry with silicon reverse bonds to happen with selenium or tellurium, but this, according to Bernstein, is unlikely. On Earth, such organisms would multiply very slowly, and our biochemistry would not interfere with each other in any way. They, however, could slowly eat up our cities, but "a jackhammer could be applied to them."
Other biochemical options
Basically, there have been quite a few proposals for life systems based on anything other than carbon. Like carbon and silicon, boron also tends to form strong covalent molecular bonds, forming different structural variants of the hydride, in which boron atoms are linked by hydrogen bridges. Like carbon, boron can bind with nitrogen to form compounds with chemical and physical properties similar to alkanes, the simplest organic compounds. The main problem with boron-based life is that it is a fairly rare element. Boron-based life will be most appropriate in an environment that is cold enough for liquid ammonia, then chemical reactions will be more controlled.
Another possible life form that has received some attention is arsenic-based life. All life on Earth is made up of carbon, hydrogen, oxygen, phosphorus and sulfur, but in 2010 NASA announced that it had found the bacteria GFAJ-1, which could incorporate arsenic instead of phosphorus into the cellular structure without any consequences for itself. GFAJ-1 lives in the arsenic-rich waters of Lake Mono in California. Arsenic is poisonous to any living creature on the planet, except for a few microorganisms that normally carry it or breathe it. GFAJ-1 is the first time the body has incorporated this element as a biological building block. Independent experts diluted this claim a little when they found no evidence of arsenic included in DNA, or even any arsenates. Nevertheless, interest has flared up in possible biochemistry based on arsenic.
Ammonia has also been put forward as a possible alternative to water for building life forms. Scientists have suggested the existence of a biochemistry based on nitrogen-hydrogen compounds that use ammonia as a solvent; it could be used to create proteins, nucleic acids and polypeptides. Any ammonia-based life must exist at low temperatures at which ammonia takes on a liquid form. Solid ammonia is denser than liquid ammonia, so there is no way to stop it from freezing when it gets cold. For unicellular organisms, this would not be a problem, but it would cause chaos for multicellular organisms. Nevertheless, there is the possibility of the existence of unicellular ammonia organisms on the colder planets of the solar system, as well as on gas giants like Jupiter.
Sulfur is believed to have served as the basis for the onset of metabolism on Earth, and known organisms that metabolize sulfur instead of oxygen exist in extreme conditions on Earth. Perhaps in another world, sulfur-based life forms could gain an evolutionary advantage. Some people think that nitrogen and phosphorus could also take the place of carbon under very specific conditions.
Memetic life
Richard Dawkins believes that the basic principle of life sounds like this: "All life develops thanks to the mechanisms of survival of reproducing creatures." Life must be able to reproduce (with some assumptions) and be in an environment where natural selection and evolution will be possible. In his book The Selfish Gene, Dawkins noted that concepts and ideas are generated in the brain and disseminated among people through communication. In many ways, this resembles the behavior and adaptation of genes, which is why he calls them "memes." Some people compare the songs, jokes and rituals of human society to the first stages of organic life - free radicals floating in the ancient seas of the Earth. The creations of the mind reproduce, evolve and struggle to survive in the realm of ideas.
Similar memes existed before humanity, in the social calls of birds and the learned behavior of primates. As humanity became able to think abstractly, memes were further developed, governing tribal relations and forming the basis for the first traditions, culture and religion. The invention of writing further pushed the development of memes, as they were able to spread in space and time, transmitting memetic information in a similar way to how genes transmit biological information. For some, this is a pure analogy, but others believe that memes represent a unique, albeit slightly rudimentary and limited form of life.
Some went even further. Georg van Driem developed the theory of "symbiosism", which implies that languages are life forms in themselves. Old linguistic theories considered language to be something of a parasite, but van Driem believes that we live in collaboration with the memetic entities that inhabit our brains. We live in a symbiotic relationship with linguistic organisms: without us they cannot exist, and without them we are no different from apes. He believes that the illusion of consciousness and free will spilled out from the interaction of animal instincts, hunger and lust of a human carrier and a linguistic symbiont reproduced with the help of ideas and meanings.
XNA based synthetic life
Life on Earth is based on two information-carrying molecules, DNA and RNA, and scientists have long wondered if other similar molecules could be created. While any polymer can store information, RNA and DNA represent heredity, the encoding and transmission of genetic information, and are able to adapt over time through evolution. DNA and RNA are chains of nucleotide molecules consisting of three chemical components - phosphate, a five-carbon sugar group (deoxyribose in DNA or ribose in RNA) and one of five standard bases (adenine, guanine, cytosine, thymine, or uracil).
In 2012, a group of scientists from England, Belgium and Denmark was the first in the world to develop xenonucleic acid (XNA, XNA), synthetic nucleotides that functionally and structurally resemble DNA and RNA. They were developed by replacing the sugar groups of deoxyribose and ribose with various substitutes. Such molecules have been made before, but for the first time in history they were able to reproduce and evolve. In DNA and RNA, replication occurs by polymerase molecules that can read, transcribe, and reverse transcribe normal nucleic acid sequences. The group developed synthetic polymerases that created six new genetic systems: HNA, CeNA, LNA, ANA, FANA, and TNA.
One of the new genetic systems, HNA, or hexitonucleic acid, was robust enough to store the right amount of genetic information that could serve as the basis for biological systems. Another, threosonucleic acid, or TNA, turned out to be a potential candidate for the mysterious primary biochemistry that reigned at the dawn of life.
There are many potential uses for these advances. Further research could help develop better models for the emergence of life on Earth and will have implications for biological inventions. XNA has therapeutic applications because it is possible to create nucleic acids to treat and bind to specific molecular targets that do not deteriorate as quickly as DNA or RNA. They can even form the basis of molecular machines or, in general, an artificial life form.
But before this is possible, other enzymes must be developed that are compatible with one of the XNAs. Some of them were already developed in the UK at the end of 2014. There is also the possibility that XNA can harm RNA / DNA organisms, so safety must come first.
Chromodynamics, Weak Nuclear Force, and Gravitational Life
In 1979, scientist and nanotechnologist Robert Freitas Jr. proposed a possible non-biological life. He stated that the possible metabolism of living systems is based on four fundamental forces - electromagnetism, strong nuclear force (or quantum chromodynamics), weak nuclear force, and gravity. Electromagnetic life is the standard biological life we have on Earth.
Chromodynamic life could be based on a strong nuclear force, which is considered the strongest of the fundamental forces, but only over extremely short distances. Freitas theorized that such a medium might be possible in a neutron star, a heavy rotating object 10-20 kilometers in diameter with the mass of a star. With an incredible density, powerful magnetic field and gravity 100 billion times stronger than on Earth, such a star would have a core with a 3 km crust of crystalline iron. Beneath it there would be a sea with incredibly hot neutrons, various nuclear particles, protons and atomic nuclei, and possible neutron-rich "macro-nuclei". These macronuclei, in theory, could form large supernuclei, analogous to organic molecules, neutrons would act as the equivalent of water in a bizarre pseudobiological system.
Freitas saw life forms based on weak nuclear interactions as unlikely, since weak forces operate only in the subnuclear range and are not particularly strong. As beta radioactive decay and free decay of neutrons often show, weak interactions life forms could exist with careful control of weak interactions in their environment. Freitas envisioned creatures made up of atoms with excess neutrons that become radioactive when they die. He also suggested that there are regions of the Universe where a weak nuclear force is stronger, which means that the chances of such life emerging are higher.
Gravitational beings can exist too, since gravity is the most abundant and effective fundamental force in the universe. Such creatures could receive energy from gravity itself, receiving unlimited power from collisions of black holes, galaxies, and other celestial objects; smaller creatures from the rotation of the planets; the smallest - from the energy of waterfalls, wind, tides and ocean currents, possibly earthquakes.
Dust and Plasma Life Forms
Organic life on Earth is based on molecules with carbon compounds, and we have already figured out possible compounds for alternative forms. But in 2007, an international group of scientists led by V. N. Tsytovich from the Institute of General Physics of the Russian Academy of Sciences documented that, under the right conditions, particles of inorganic dust can collect into spiral structures, which will then interact with each other in a manner inherent to organic chemistry. This behavior is also born in the plasma state, the fourth state of matter after solid, liquid and gaseous, when electrons are detached from atoms, leaving a mass of charged particles.
Tsytovich's group found that when electron charges are separated and the plasma is polarized, the particles in the plasma self-organize into spiral structures like a corkscrew, electrically charged, and attract each other. They can also divide by making copies of original structures, like DNA, and induce charges in their neighbors. According to Tsytovich, “these complex, self-organizing plasma structures meet all the necessary requirements to be considered candidates for inorganic living matter. They are autonomous, they reproduce and they evolve."
Some skeptics believe such claims are more attention grabbing than serious scientific claims. Although helical structures in plasma may resemble DNA, similarity in shape does not necessarily imply similarity in function. Moreover, the fact that the spirals reproduce does not mean the potential for life; clouds do it too. Even more disheartening, much of the research has been done on computer models.
One of the participants in the experiment also reported that although the results did resemble life, in the end they were "just a special form of plasma crystal." And yet, if inorganic particles in plasma can grow into self-replicating, evolving life forms, they could be the most abundant form of life in the universe, thanks to the ubiquitous plasma and interstellar dust clouds throughout the cosmos.
Inorganic chemical cells
Professor Lee Cronin, a chemist at the College of Science and Engineering at the University of Glasgow, dreams of creating living cells from metal. He uses polyoxometallates, a series of metal atoms bonded to oxygen and phosphorus, to create cell-like bubbles, which he calls "inorganic chemical cells," or iCHELLs (an acronym that can be translated as "neocheleta").
Cronin's group began by creating salts from negatively charged ions of large metal oxides bound to a small positively charged ion like hydrogen or sodium. A solution of these salts is then injected into another saline solution full of large positively charged organic ions bound to small negatively charged ones. The two salts meet and exchange parts, so that large metal oxides partner with large organic ions, forming a kind of bubble that is impervious to water. By modifying the backbone of the metal oxide, the bubbles can acquire the properties of biological cell membranes that selectively pass and release chemicals from the cell, which could potentially allow the same type of controlled chemical reactions that occurs in living cells.
The team has also made bubbles within bubbles by mimicking the internal structures of biological cells and has made progress in creating an artificial form of photosynthesis that could potentially be used to create artificial plant cells. Other synthetic biologists point out that such cells may never become alive until they have a system of replication and evolution like DNA. Cronin does not lose hope that further development will bear fruit. Possible applications of this technology also include the development of materials for solar fuel devices and, of course, medicine.
According to Cronin, "the main goal is to create complex chemical cells with living properties that can help us understand the development of life and follow the same path to bring new technologies based on evolution into the material world - a kind of inorganic living technologies."
Von Neumann probes
Machine-based artificial life is a fairly common idea, almost banal, so let's just look at von Neumann probes so as not to bypass it. They were first invented in the middle of the 20th century by the Hungarian mathematician and futurist John von Neumann, who believed that in order to reproduce the functions of the human brain, a machine must have mechanisms of self-control and self-healing. So he came up with the idea of creating self-reproducing machines, based on observations of the increasing complexity of life in the process of reproduction. He believed that such machines could become a kind of universal constructor that could allow not only to create complete replicas of itself, but also to improve or change versions, thereby carrying out evolution and increasing complexity over time.
Other futurists like Freeman Dyson and Eric Drexler quickly applied these ideas to space exploration and created the von Neumann probe. Sending a self-replicating robot into space may be the most efficient way to colonize a galaxy, as it can capture the entire Milky Way in less than one million years, even at the speed of light.
As Michio Kaku explained:
“The von Neumann probe is a robot designed to reach distant stellar systems and create factories that will build thousands of copies of themselves. A dead moon, not even a planet, could be an ideal destination for von Neumann probes, as it will make it easier to land and take off from those moons, and also because the moons do not have erosion. The probes could live off the land, mining iron, nickel and other raw materials to build robotic factories. They would create thousands of copies of themselves, which would then disperse in search of other star systems."
Over the years, various versions of the basic idea of the von Neumann probe have been devised, including exploration and exploration probes for quietly exploring and observing extraterrestrial civilizations; communication probes scattered throughout space to better pick up alien radio signals; working probes for the construction of supermassive space structures; colonizing probes that will conquer other worlds. There may even be guiding probes that will take young civilizations into space. Alas, there may be berserk probes, whose task will be to destroy traces of any organic matter in space, followed by the construction of police probes that will reflect these attacks. Given that von Neumann probes can become a kind of space virus, we should be careful when developing them.
Gaia's hypothesis
In 1975, James Lovelock and Sidney Upton co-wrote an article for the New Scientist entitled "Finding Gaia." Adhering to the traditional view that life originated on Earth and flourished due to the right material conditions, Lovelock and Upton suggested that life thus took an active role in maintaining and determining the conditions for its survival. They suggested that all living matter on Earth, in the air, oceans and on the surface is part of a single system that behaves like a superorganism that is able to adjust the temperature on the surface and the composition of the atmosphere in a way necessary for survival. They named this system Gaia, after the Greek goddess of the earth. It exists to maintain homeostasis, thanks to which the biosphere can exist on earth.
Lovelock has been working on the Gaia hypothesis since the mid-1960s. The basic idea is that the Earth's biosphere has a number of natural cycles, and when one goes awry, others compensate for it in a way that maintains vital capacity. This could explain why the atmosphere is not made entirely of carbon dioxide or why the seas are not too salty. Although volcanic eruptions made the early atmosphere predominantly carbon dioxide, nitrogen-producing bacteria and plants emerged that produce oxygen through photosynthesis. Millions of years later, the atmosphere has changed in our favor. While rivers carry salt to the oceans from rocks, the salinity of the oceans remains stable at 3.4% as salt seeps through cracks in the ocean floor. These are not conscious processes, but the result of feedback,which keeps the planets in habitable equilibrium.
Other evidence includes that if it weren't for biotic activity, methane and hydrogen would disappear from the atmosphere in just a few decades. In addition, despite a 30% increase in the sun's temperature over the past 3.5 billion years, the average global temperature has staggered by only 5 degrees Celsius, thanks to a regulatory mechanism that removes carbon dioxide from the atmosphere and traps it in fossilized organic matter.
Initially, Lovelock's ideas were met with ridicule and accusations. Over time, however, Gaia's hypothesis influenced the ideas about the Earth's biosphere, helping to form their integral perception in the scientific world. Today, Gaia's hypothesis is respected rather than accepted by scientists. Rather, it is a positive cultural framework within which scientific research on the Earth as a global ecosystem should be conducted.
Paleontologist Peter Ward developed the competitive Medea hypothesis, named after the mother who killed her children, in Greek mythology, the main idea of which is that life is inherently self-destructive and suicidal. He points out that historically most of the mass extinctions were caused by life forms such as microorganisms or hominids in pants, which severely injure the Earth's atmosphere.